1. FIELD OF THE INVENTION
The present invention relates to a signal holding circuit which, when a pulsive noise is included in a received signal of a receiver so that the pulsive noise is superimposed on an audio signal derived from a detection circuit of the receiver, holds a level of the audio signal during a period of superposition to a constant value at a moment immediately before the occurrence of the pulsive noise to suppress the pulsive noise.
2. DESCRIPTION OF THE PRIOR ART
A receiver, particularly a vehicle-mounted receiver is subjected to disturbance of signal reception by a pulsive noise which occurs by an internal combustion engine of an automobile or the like. U.S. Pat. Nos. 3,739,285 and 3,689,845 disclose circuits for suppressing such a pulsive noise. More particularly, in the circuits disclosed in those patents, a gate circuit including a gate transistor is provided between a detection circuit and an output circuit, a pulsive noise included in a received signal is detected by a pulsive noise detection circuit, the detected pulse is shaped by a pulse shaping circuit to produce a control pulse corresponding to a duration of the pulsive noise which control pulse is applied to a gate of the gate transistor e.g. a field effect transistor, of the gate circuit to cut off the source-drain of the gate transistor, and a hold capacitor is connected to the drain of the gate transistor so that when the control pulse is not applied and the gate transistor is conducting, an audio signal derived from the detection circuit and applied to the source is supplied to the hold capacitor and when the control pulse is applied and the gate transistor is cut off, the audio signal is discharged whereby a potential at the drain of the gate transistor, that is, a level of the audio signal is held at a level at a moment immediately before the occurrence of the pulsive noise.
In such a circuit where the control pulse is applied to the gate of the gate transistor to control the gate potential of the gate transistor for blocking the source-drain of the gate transistor to suppress the pulsive noise, there exists a drawback in that the gate control pulse appears at the drain or the output of the gate transistor through the gate-drain of the gate transistor. This is because, since the gate transistor is normally saturated during the reception of the signal, the gate transistor assumes an active condition even momentarily when the gate potential of the gate transistor is controlled to render itself non-conductive and the gate transistor acts as a source follower transistor during that period. A similar problem occurs when a bipolar transistor is used as the gate transistor. Accordingly, in that circuit, a compensation circuit means for eliminating the sprious control pulse is additionally required.